1. Field of the Invention
The present invention relates to a novel silica sol and a method for preparing the same. Specifically, the novel silica sol is characterized by a particular shape of colloidal silica particle and displays an excellent filming or coating property after being coated and dried on a solid article and therefore it is utilized as a coating composition and in other various fields.
2. Description of Related Art
A silica sol has a general property that it finally gels from its low viscosity state via its high viscosity state. Accordingly, for silica sol products having the same SiO.sub.2 content, ones having a lower viscosity are evaluated to have a higher stability than others having a higher viscosity. In addition, silica sol containing colloidal silica particles with a more spherical shape has been known to have a lower viscosity. Under this situation, there have heretofore been various proposals for efficiently preparing spherical colloidal silica sols, but there is no proposal up to the present for improving the characteristic of a silica sol by controlling the shape of the colloidal silica particles as dispersed in the silica sol to be non-spherical.
Regarding the shape of the conventional colloidal silica particles, U.S. Pat. No. 2,680,721 mentions three typical types of particles in the drawings as attached thereto. The first is a spherical one as mentioned above, which is shown in FIG. 1 thereof; the second is a non-spherical one having a ratio of the major axis to the minor axis of almost from 2 to 3, which is shown in FIG. 2, bottom view thereof; and the third is an amorphous one, which is shown in FIG. 3, bottom view thereof. The random-shaped particles of the third type are ones resulting from growth of the fragments formed by cleavage of the chain of a three-dimensional network structure of particles of very small size derived from chain-like linkage of smaller silica particles, as so explained in the said U.S. Pat. No. 2,680,721. When attention is paid to one particle of such type, it is noted that it has a non-spherical shape but the shape is not controlled.
The above-mentioned U.S. Pat. No. 2,680,721 has disclosed a method of forming a silica sol comprising spherical colloidal silica particles where a monovalent base is added to spherical silica sol having a diameter of 5 m.mu. (milli micron) or more so that the pH value of the sol may be within the range of from 7 to 10.5 and the sol is heated at 160.degree. to 300.degree. C. in the absence of electrolytes.
U.S. Pat. No. 2,900,348 has disclosed a method of forming a silica sol where a gel formed by adding an acid to an aqueous solution of water glass is washed with water, an alkali is added to the gel so that the gel may have a pH value of from 9 to 9.5, and the gel is then heated at 95.degree. to 100.degree. C. The said method is a so-called peptization method, and the silica sol prepared by the method has a shape of the above-mentioned second or third type.
Silica sols comprising spherical colloidal silica particles having a particle diameter of from 4 to 150 m.mu. are highly stable and are therefore used in various fields. However, the spherical particles which give good dispersibility to the sol would often have some practical problems in that they frequently cause cracking of the film to be formed from the silica solcontaining composition and migration of colloidal silica particles is generated in a composition comprising the silica sol and ceramic fibers when the composition is dried so that dusting on the surface of the dried article is frequently caused. In order to solve such problems, a means of adding some other components to the silica sol has been employed, but sufficient improvement to the problems has not been attained.
The silica sol obtainable by the said conventional peptization method does not have a sufficient stability, and, as the case may be, it would often form a precipitate of silica during storage thereof. Although the colloidal silica particles in the said sol are non-spherical, they also have the same problems as those in the case of the silica sol comprising the above-mentioned spherical colloidal silica particles.